Hybrid electric vehicles may include a generator that is used to control the speed of an engine. In response to a power demand, the engine speed and torque may be selected based on a calibratable map, which may be referred to as the energy management system (EMS) map. The EMS map is calibrated to achieve desired fuel economy and various other considerations, including performance, emissions, and vehicle noise, vibration, and harshness. The EMS map is generally calibrated based on sea level engine performance.
At some environmental conditions, such as at increased altitudes or high ambient temperatures, the torque availability of the engine may be reduced at some or all engine speeds. At such conditions, the engine may be incapable of supplying the torque specified by the EMS map at the target speed, and so the engine will not deliver the requested power at the EMS target speed. The engine speed should be increased to a point where the engine may deliver sufficient torque to satisfy the power demand.
A prior art method for controlling the engine in such conditions involves increasing the engine power request until the desired engine power is provided, as illustrated in FIG. 1. A representation of an EMS curve is shown, providing a target engine speed and engine torque for a given power demand. Here, the desired power corresponds with an EMS speed and torque as indicated at point A. A controller will command the engine to produce the EMS speed associated with point A. However, because of an environmental condition such as increased altitude, the maximum engine torque is reduced at the EMS speed, as indicated by point A′. Consequently the power supplied will be less than the desired power. In response to the power shortfall, the controller will increase the power request. As the power request increases, the commanded engine speed will increase along the EMS curve until point B is reached. At point B, the EMS speed and the reduced maximum torque correspond with point B′. The engine power at point B′ is equal to the EMS power at point A, and thus the desired power is satisfied. Because the desired power is satisfied, the controller will maintain the commanded engine speed at B until the desired engine power changes.
While this method may be suitable for some applications, it is relatively time-intensive, because engine torque production is delayed from an engine torque request due to effects such as manifold filling and combustion delays. Consequently, any feedback mechanism must wait until torque is produced before determining whether there is a power shortfall and further increasing power. In addition, this method increases engine speed only until the delivered engine power equals the desired engine power. As a result, the engine will operate at its maximum torque limit. During steady-state operation this may be undesirable, because some level of engine torque reserve should be maintained in case the accelerator pedal is further depressed (“tip in”) and to provide sufficient vacuum for EGR and canister purge. Furthermore, if the accelerator pedal is released (“tip out”), the actual engine speed may take some time to reach the new, lower, EMS target speed. During this time, the additional engine power requested to compensate for the reduced engine torque capability may be achievable, resulting in over-production of engine power and the potential to over-charge the battery.
Another prior art method involves an engine controller computing a table of maximum achievable engine torque at pre-specified engine speeds. These computed values were converted into a table containing maximum engine power as a function of engine speed. An interpolation is then performed on the table to determine the lowest engine speed at which a desired engine power is achievable. This process is repeated during every execution loop of the algorithm. At altitude conditions, the engine speed from this algorithm can be used to over-ride the speed from the EMS map. This method may also be unsatisfactory for some applications because it is highly processor intensive, requiring identical calculations at many different engine speeds for each execution loop.